The present invention relates to an information processing system having multilevel address translation tables configured in a hierarchic structure.
In the past, translation tables having a hierarchic structure were used to achieve address translation in many information processing systems. The hierarchic structure is adopted because it makes it possible to flexibly construct address translation tables. That is, if the hierarchic structure is not used, the translation tables must be placed in continuous areas; moreover, the tables covering the entire access range must be provided for each program. However, if the translation tables are configured in the hierarchic structure, it is sufficient to provide for each program translation tables associated with the address range to be accessed by the program. In addition, even when the access range dynamically changes, the necessary data can be easily added to the translation tables. Conventionally, the translation tables have been constructed in a two-level configuration. When the address space is desired to be expanded in this case, multilevel translation tables become necessary accordingly. For example, if the upper-limit value of the address is increased from 2.sup.31 -1 to 2.sup.63 -1, translation tables which are structured in about five-level configuration are necessary.
Since such an expansion of the upper-limit value of the virtual address greatly affects the hardware and software of the information processing system, the expansion is not gradually executed in a step-by-step fashion; namely, the upper-limit value is changed to a considerably great value in ordinary cases. However, the address space which is actually required increases only gradually. Especially, conventional programs can operate sufficiently within the conventional address space.
For the processing speed, consequently, it is not desirable that the address translation is carried out using all translation tables for such programs which do not have a large address space.